In thrust type 35 mm film manufacturers' cassettes, such as those manufactured by Eastman Kodak Co. and Fuji Film Photo Ltd., a leading portion of a film strip is thrust from the confines of a loaded film cassette onto a take-up spool rather than having to manually extract the protruding leading portion, as is common to other conventional non-thrusting film cassettes. This is particularly advantageous in that a protruding portion of the film is not required and that a user does not have to take the time to manually extend the leading portion of the film for cinching to a take-up spool prior to image capture.
Another advantage realized using thrust type film cassettes is that a partially exposed film can be rewound into the cassette, and be extracted from the confines of the camera. The cassette can then be reloaded into a camera having particular logic for allowing the film strip to be thrust and properly advanced to the next exposable frame prior to image capture. The features of rewinding a partially exposed film back into the cartridge and for subsequent exposure of the previously unexposed film is referred to as mid-roll interrupt (MRI).
In commonly assigned U.S. Pat. No. 4,994,828, there is described an apparatus for preventing an exposed film thrusting cartridge from being loaded into a film chamber of the camera. A blocking member, having an engageable end is biased by a leaf spring into the film loading chamber of a camera featuring drop-in loading. The blocking member cooperates with the status indicator of the film cartridge to prevent the cartridge from being loaded in the film chamber if the cartridge contains either exposed or partially exposed film. If the film cartridge, however, contains fresh film, or in cameras having MRI logic partially exposed film, the blocking member is pivoted out of the chamber and the cartridge can be loaded.
In commonly assigned and concurrently filed U.S. Ser. No. 08/386,792, filed by J. David Cocca, Paul L. Taillie, and Beth A. O'Leary, a cartridge sensing element is separately provided adjacent the blocking member to prevent the double exposure of a loaded film cartridge after the film contained therein has been either partially or fully exposed and then rewound into the cartridge.
In a preferred embodiment, the cartridge sensing element is a resettable electrical switch which is biased in a first position by another leaf spring cooperates with the blocking member so that when urged out of the chamber by an unblocked cassette, a portion of the blocking member opens the switch, breaking an electrical connection with a contact surface. The opening of the switch is sensed by a controller which enables a film advancing means to load film from the unblocked cassette and prevents the film from being reloaded until the switch undergoes another close to open transition.
It is desirable to attempt to combine the features of the sensing element and the blocking member to provide an efficient double exposure prevention apparatus for film cartridges, particularly thrust type cartridges.